1. Field of the Invention
This invention relates to a gamma voltage generating apparatus for a display device, and more particularly to a gamma voltage generating apparatus that is adaptive for reducing the number of parts to simplify a structure thereof.
2. Description of the Related Art
Recently, there have been highlighted various flat panel display devices reduced in weight and bulk that is capable of eliminating disadvantages of a cathode ray tube (CRT). Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an electro-luminescence (EL) display, etc.
The EL display in such display devices is a self-luminous device capable of light-emitting a phosphorous material by a re-combination of electrons with holes. The EL display device is generally classified into an inorganic EL device using an inorganic compound as the phosphorous material and an organic EL using an organic compound as the phosphorous material. The EL display has the same advantage as the CRT in that it has a faster response speed than a passive-type light-emitting device requiring a separate light source. Further, the EL display device has many advantages of a low voltage driving, a self-luminescence, a thin-thickness, a wide viewing angle, a fast response speed and a high contrast, etc. such that it can be highlighted into a post-generation display device.
FIG. 1 is a section view showing a general organic EL structure for explaining a light-emitting principle of the EL display device.
Referring to FIG. 1, the organic EL device is comprised of an electron injection layer 4, an electron carrier layer 6, a light-emitting layer 8, a hole carrier layer 10 and a hole injection layer 12 that are sequentially disposed between a cathode 2 and an anode 14.
If a voltage is applied between a transparent electrode, that is, the anode 14 and a metal electrode, that is, the cathode 2, then electrons produced from the cathode 2 are moved, via the electron injection layer 4 and the electron carrier layer 6, into the light-emitting layer 8 while holes produced from the anode 14 are moved, via the hole injection layer 12 and the hole carrier layer 10, into the light-emitting layer 10. Thus, the electrons and the holes fed from the electron carrier layer 6 and the hole carrier layer 10, respectively, are collided at the light-emitting layer to be recombined to thereby generate a light, and this light is emitted, via the transparent electrode (i.e., the anode 14), into the exterior to thereby display a picture. Since a light-emitting brightness of the organic EL device is in proportion to a supply current rather than being in proportion to a voltage loaded on each end of the device, the anode 14 is generally connected to a positive current source.
FIG. 2 schematically shows a general EL display device.
Referring to FIG. 2, the EL display device includes an EL panel 20 having EL cells 28 arranged at intersections between scan electrode lines SL and data electrode lines DL, a scan driver 22 for driving the scan electrode lines SL, a data driver 24 for driving the data electrode lines DL, and a gamma voltage generator 26 for supplying a plurality of gamma voltages to the data driver 24.
Each of EL cells 28 is selected when a scanning pulse is applied to the scan electrode line SL as a cathode to thereby generate a light corresponding to a pixel signal, that is, a current signal applied to the data electrode line DL as an anode. Each EL cell 28 can be equivalently expressed as a diode connected between the data electrode line DL and the scan electrode line SL. Each EL cell 28 is light-emitted when a negative scanning pulse to the scan electrode line SL and, at the same time, a positive current according to a data signal is applied to the data electrode line DL to thereby load a forward current. Otherwise, the EL cells 28 included in the unselected scan line are supplied with a backward current to thereby be not light-emitted. In other words, forward electric charges are charged in the emitting EL cells 28 while backward electric charges are charged in the non-emitting EL cells 28.
The scan driver 22 applies a negative scanning pulse to a plurality of scan electrode lines SL on a line-sequence basis.
The data driver 24 converts a digital data signal inputted from the exterior thereof into an analog data signal using a gamma voltage from the gamma voltage generator 26. Further, the data driver 24 applies the analog data signal to the data lines DL whenever the scanning pulse is supplied.
As mentioned above, the conventional EL display device applies a current proportional to an input data to each EL cell 28 to light-emit each EL cell 28, thereby displaying a picture. The EL cells 28 consist of a red (R) cell having a red phosphorous material, a green (G) cell having a green phosphorous material and a blue (B) cell having a blue phosphorous material. The three R, G and B cells are combined to thereby implement a color for one pixel. Herein, the R, G and B phosphorous materials have different light-emission efficiency. In other words, when data signals having the same level are applied to the R, G and B cells, brightness levels of the R, G and B cells become different from each other. Thus, gamma voltages are set differently for each R, G and B cell with respect to the same brightness for the sake of white balance of the R, G and B cells. Accordingly, the gamma voltage generator 26 for supplying gamma voltages to the data driver 24 generates a gamma voltage for each R, G and B cell.
FIG. 3 is a detailed circuit diagram of the gamma voltage generator shown in FIG. 2.
Referring to FIG. 3, the conventional gamma voltage generator includes an R gamma voltage generator 32, a G gamma voltage generator 34 and a B gamma voltage generator 36 in order to supply gamma voltage for each R, G and B cell.
The R gamma voltage generator 32 has voltage-dividing resistors r_R1, r_R2 and r_R3 connected, in series, between a supply voltage source VDD and a ground voltage source GND. Herein, voltages from common nodes n1 and n2 of the voltage-dividing resistors r_R1, r_R2 and r_R3 are inputted to the data driver 24 as gamma voltages. At this time, a high gray level of R gamma voltage VH_R is generated on a basis of the following equation (1) while a low gray level of R gamma voltage VL_R is generated on a basis of the following equation (2).
                    VH_R        ⁢                                  ⁢                  (                      a            ⁢                                                  ⁢            low            ⁢                                                  ⁢            gray            ⁢                                                  ⁢            level            ⁢                                                  ⁢            of            ⁢                                                  ⁢            R            ⁢                                                  ⁢            gamma            ⁢                                                  ⁢            voltage                    )                =                                            r_R2              +              r_R3                                      r_R1              +              r_R2              +              r_R3                                *          VDD                                    (        1        )                                VL_R        ⁢                                  ⁢                  (                      a            ⁢                                                  ⁢            high            ⁢                                                  ⁢            gray            ⁢                                                  ⁢            level            ⁢                                                  ⁢            of            ⁢                                                  ⁢            R            ⁢                                                  ⁢            gamma            ⁢                                                  ⁢            voltage                    )                =                              r_R3                          r_R1              +              r_R2              +              r_R3                                *          VDD                                    (        2        )            
The G gamma voltage generator 34 has voltage-dividing resistors r_G1, r_G2 and r_G3 connected, in series, between the supply voltage source VDD and the ground voltage source GND. Herein, voltages from common nodes n3 and n4 of the voltage-dividing resistors r_G1, r_G2 and r_G3 are inputted to the data driver 24 as gamma voltages. At this time, a high gray level of G gamma voltage VH_G is generated on a basis of the following equation (3) while a low gray level of G gamma voltage VL_G is generated on a basis of the following equation (4).
                              VH_G          ⁢                                          ⁢                      (                          a              ⁢                                                          ⁢              low              ⁢                                                          ⁢              gray              ⁢                                                          ⁢              level              ⁢                                                          ⁢              of              ⁢                                                          ⁢              G              ⁢                                                          ⁢              gamma              ⁢                                                          ⁢              voltage                        )                          =                                            r_G2              +              r_G3                                      r_G1              +              r_G2              +              r_G3                                *          VDD                                    (        3        )                                          VL_G          ⁢                                          ⁢                      (                          a              ⁢                                                          ⁢              high              ⁢                                                          ⁢              gray              ⁢                                                          ⁢              level              ⁢                                                          ⁢              of              ⁢                                                          ⁢              G              ⁢                                                          ⁢              gamma              ⁢                                                          ⁢              voltage                        )                          =                              r_G3                          r_G1              +              r_G2              +              r_G3                                *          VDD                                    (        4        )            
The B gamma voltage generator 36 has voltage-dividing resistors r_B1, r_B2 and r_B3 connected, in series, between the supply voltage source VDD and the ground voltage source GND. Herein, voltages from common nodes n5 and n6 of the voltage-dividing resistors r_B1, r_B2 and r_B3 are inputted to the data driver 24 as gamma voltages. At this time, a high gray level of B gamma voltage VH_B is generated on a basis of the following equation (5) while a low gray level of B gamma voltage VL_B is generated on a basis of the following equation (6).
                              VH_B          ⁢                                          ⁢                      (                          a              ⁢                                                          ⁢              low              ⁢                                                          ⁢              gray              ⁢                                                          ⁢              level              ⁢                                                          ⁢              of              ⁢                                                          ⁢              B              ⁢                                                          ⁢              gamma              ⁢                                                          ⁢              voltage                        )                          =                                            r_B2              +              r_B3                                      r_B1              +              r_B2              +              r_B3                                *          VDD                                    (        5        )                                          VL_B          ⁢                                          ⁢                      (                          a              ⁢                                                          ⁢              high              ⁢                                                          ⁢              gray              ⁢                                                          ⁢              level              ⁢                                                          ⁢              of              ⁢                                                          ⁢              B              ⁢                                                          ⁢              gamma              ⁢                                                          ⁢              voltage                        )                          =                              r_B3                          r_B1              +              r_B2              +              r_B3                                *          VDD                                    (        6        )            
Meanwhile, the conventional EL display device further includes a gamma voltage generator for each mode as shown in FIG. 4 and FIG. 5 such that brightness is changed in correspondence with various environments. Herein, resistors included the gamma voltage generator for each mode have resistance values established such that brightness corresponding to an environment (light), such as night, noon, the exterior, the interior and the like, can be generated.
For instance, the R gamma voltage generator 32 of the second mode gamma voltage generator shown in FIG. 4 includes voltage-dividing resistors r_R4, r_R5 and r_R6 connected, in series, between the supply voltage source VDD and the ground voltage source GND. Herein, resistance values of the voltage-dividing resistors r_R4, r_R5 and r_R6 are set differently from those of the voltage-dividing resistors r_R1, r_R2 and r_R3 included in the R gamma voltage generator 32 shown in FIG. 3. Thus, gamma voltage values generated at the second mode gamma voltage generator are set differently from gamma voltage values generated at the R gamma voltage generator 32 shown in FIG. 3. These gamma voltage values are supplied to the EL display device in correspondence with an environment, thereby allowing the EL display device to generate an optimum brightness corresponding to an external environment. Herein, resistance values of voltage-dividing resistors r_R7, r_R8 and r_R9 are set differently from those of the voltage-dividing resistors r_R1, r_R2, r_R3, r_R4, r_R5 and r_R6 included in the R gamma voltage generators 32 shown in FIG. 3 and FIG. 4.
However, the gamma voltage generator corresponding to each mode in this manner must generates a high gray level of R gamma voltage VH_R and a low gray level of R gamma voltage VL_R applied to the R cell, a high gray level of G gamma voltage VH_G and a low gray level of R gamma voltage VL_G applied to the G cell, and a high gray level of B gamma voltage VH_B and a low gray level of B gamma voltage VL_B applied to the B cell. In other words, the gamma voltage generator must generate all of a high gray level of gamma voltage VH_R, VH_G and VH_B and a low gray level of gamma voltages VL_R, VL_G and VL_B. To this end, since the R, G and B gamma voltage generators 32, 34 and 36 of the gamma voltage generator generates a high gray level of gamma voltage VH_R, VH_G and VH_B and a low gray level of gamma voltages VL_R, VL_G and VL_B among three resistors connected in series, nine resistors are provided for each mode. Thus, when three modes are used, the conventional gamma voltage generator must be provided with total 27 resistors. Accordingly, the conventional EL display device has a problem in that many different parts are provided at the module to have a complicate structure.